Among the liquid crystal display devices, active matrix liquid crystal display devices have been used frequently as a display device for a personal computer or as a display device for a liquid crystal television. Moreover, the size of the screen has been increasing rapidly for such applications. Further, regardless of the size of the display screen, a high definition display in the wide color reproduction range at a high image quality is required for the active matrix liquid crystal display device. These are required in order to ensure superiority in competitiveness over other display devices, such as a plasma display panel (PDP). In addition, less power consumption is required at the same time.
The active matrix color liquid crystal display device displays a full-color image by providing red (R), green (G), and blue (B) color filters on sub-pixels forming each pixel and allowing white light from the backlight illumination device to pass through these filters. In short, a full-color image is displayed by forming each pixel by combining three sub-pixels for R, G, and B. Light conversion efficiency of the backlight illumination device is low and the rest of input power turns into heat. The heat sink portion therefore increases in size.
With the aim of not only expanding the color reproduction range but also extending the life, a backlight illumination device using a plurality of light emitting diodes (LEDs) for colors including three primary colors, red light (R light), green light (G light), and blue light (B light), is now put into practical use instead of the conventional backlight illumination device using a cold cathode fluorescent tube. Because the light emitting diode can expand the color reproduction range in comparison with the cold cathode fluorescent tube, it is possible to achieve a liquid crystal display device with a higher image quality.
However, in a case where the LED light source emitting light of at least three colors including R light, G light, and B light is used, a luminescent color varies from one LED to another. For example, the same G light may take on a reddish luminescent color or bluish luminescent color depending on the LEDs. Also, the luminescent color from the same element may possibly vary due to factors, such as a driving current and temperature characteristics. In the case of a full-color display using LEDs for R light, G light, and B light as described above, it is difficult to maintain the chromaticity of the while level constant. Further, although it is possible to adjust the chromaticity of the white level at a point in time when a liquid crystal display device is manufactured, the white level changes with a variation in deterioration with time over a long period.
Furthermore, in a case where the LEDs are used, the luminous wavelength and a light output vary with heat generation by the LEDs. Accordingly, even when the luminance and the tones are adjusted once, the luminance and the tones may possibly vary again after the adjustment. Deterioration with time can also give rise to such variations. Under these circumstances, there has been proposed a configuration in which a semiconductor laser element suitable to a high output at higher luminance than LEDs is used as a light emitting element for at least any one of the colors among the three colors of the light emitting elements, so that variations of the characteristics can be lessened by suppressing heat generation caused by an increase of the driving current. This first example concretely describes the use of a red semiconductor laser (for example, Patent Document 1).
In the first example, however, when adopting the conventional method of using a fluorescent tube or LEDs as the light source, because the light source is disposed behind or on the side of the panel and the light source is driven in this state, the thickness and the weight including those of the liquid crystal display panel are added. Also, input power that turns into heat without being converted to light is large. Hence, the heat radiation structure is required and the thickness and the weight of this structure are added, too. The liquid crystal panel is vulnerable to heat and it has to be spaced apart from the light source (lamp and LEDs), which is a heat generator. The device is thus increased in size.
Further, the first example above describes the use of a red semiconductor laser as the light source in the backlight illumination device. However, it fails to disclose any concrete configuration or the like. Hence, with the use of the red semiconductor laser, it is possible to lessen variations of the characteristics by suppressing heat generated caused by an increase of the driving current. However, no measure is taken against a size increase of the device itself. In addition, the main configuration of the first example above is to use the LEDs as the light source, and in a case where laser light sources for three colors including R light, G light, and B light and a white light source are used as the light source, a problem that the liquid crystal display panel becomes thicker and heavier still remains.
For a liquid crystal display system constituted by combining the liquid crystal display devices as described above into a system form, there has been a need for a larger screen and a higher image quality as it is now used not only as a display device for a personal computer but also as a television. Such a liquid crystal display system uses a planar light source device that illuminates the liquid crystal display panel from behind. A cold cathode fluorescent tube is often used as the light source of the planar light source device. In the case of a method adopting a cold cathode fluorescent tube, however, there is a problem that the display performance of the liquid crystal display system is deteriorated by heat generated from the cold cathode fluorescent tube. Also, there has been a need for a further higher image quality arising from an increasing demand as a television receiver in recent years, and the configuration using light emitting diodes (herein after, abbreviated as LEDs) as the light source is coming into practical use.
In order to avoid adverse influences of heat generation, there has been proposed a liquid crystal display system configured in such a manner that the liquid crystal display panel is illuminated by a backlight formed of a light source having, for example, a cold cathode fluorescent tube, a first light guide body forming a planar light source and having a wedge-shaped cross section, a second light guide body provided to the end face portion of the first light guide body to supply illumination light to the first light guide body, and an optical fiber connecting the light source and the second light guide body (for example, see Patent Document 2). Different from the conventional configuration, the second example prevents influences of heat generation from the cold cathode fluorescent tube and noises generated by application of a high-frequency voltage by connecting the cold cathode fluorescent tube and the first light guide body with the optical fiber.
Also, there has been proposed a planar light source device using a light guide plate having a wedge-shaped cross section in order to achieve a reduction of the device both in size and weight by reducing an invalid region inside the light guide plate even when a point source like LEDs is used (for example, see Patent Document 3). The planar light source device using the LEDs as in the third example can achieve satisfactory color reproducibility and a high image quality in comparison with those using the cold cathode fluorescent tube.
In addition, a planar light source device that achieves a higher image quality by using not only the LEDs for red light (R light), blue light (B light), and green light (G light) but also LEDs emitting light of other colors is coming into practical use. Further, a planar light source device in which a part of the LEDs are replaced with a semiconductor laser element is being discussed. This fourth example utilizes that the semiconductor laser element has higher luminance and a higher output than the LEDs and is thereby capable of reducing driving power and enhancing the image quality (for example, see Patent Document 1).
In addition, there has been disclosed an image display system configured with the aim of achieving large-screen display on the one hand and reducing the weight of the device on the other hand (for example, see Patent Document 4). The system of the fifth example is formed of a plurality of liquid crystal modules each including a liquid crystal panel and generating a partial image making up a part of an image, a screen that displays thereon the partial images generated by a plurality of the liquid crystal modules, and a light supply portion that supplies light to the liquid crystal panels of a plurality of the liquid crystal modules. It is configured in such a manner that light is supplied to the liquid crystal modules from the light supply portion via optical fiber cables. A projection-type display system with a large screen can be achieved by the configuration as above.
According to the second example described above, it is possible to prevent influences of heat generation from the cold cathode fluorescent tube and noises generated by application of a high-frequency voltage. It is, however, relatively difficult to further improve the image quality by expanding the color reproduction range. In addition, the method of guiding light emitted from the cold cathode fluorescent tube to the light guide body via the optical fibers has a difficulty in transmitting light at high intensity. This method is therefore not applicable to the liquid crystal display system with a large screen.
Also, according to the third example described above, not only can the color reproduction range be expanded but also the life can be extended. However, in a case where the LED light source that emits light of at least three colors including R light, G light, and B light, there are problems as follows. That is, because a luminescent color varies from one LED to another, for example, the luminescent color of the same G light may take on reddish color or bluish color depending on the LEDs. Also, the luminescent color of the same LED may possibly vary by factors, such as a driving current, temperature characteristics, or deterioration with time. Accordingly, even when the luminance and the tones are adjusted at the initial stage, the luminance and the tones vary after the adjustment or in use. It is therefore difficult to maintain the chromaticity of the white level constant over a long period.
Further, because the second and third examples adopt the method in which the liquid crystal display panel is illuminated by disposing the cold cathode fluorescent tube or the LEDs behind or on the side of the panel, influences of the thickness and the weight of the light source to the overall device become more significant as the screen of the liquid crystal system becomes larger. In addition, because the heat radiation structure is also necessary, the thickness and the weight increase further. These inconveniences impose the limitations on the further reduction in thickness and weight.
The fourth example described above also discloses the configuration to lessen variations of the characteristics by suppressing heat generation caused by an increase of the driving current with the use of the semiconductor laser element suitable to a high output at higher luminance than LEDs, and the use of a red semiconductor laser as the semiconductor laser element is described concretely. However, this example neither discloses nor suggests a concrete configuration of the device or the like. Accordingly, although it is possible to lessen variances of the characteristics by suppressing heat generation caused by an increase of the driving current with the use of the red semiconductor laser, no measure is disclosed against a size increase of the device itself.
Further, the fifth example above is configured in such a manner that a plurality of liquid crystal panels are illuminated from the light supply portion via optical fibers and thereby achieves a projection-type display system capable of increasing a size of the screen on one hand and reducing the weight on the other hand. However, the fifth example is applied to the projection type and it is difficult to apply this example to a direct-view-type liquid crystal display system that is used frequently in general. Accordingly, problems arising when increasing a size of the screen on one hand and reducing the weight on the other hand still remain in the direct-view-type liquid crystal display system.
Also, the fifth example above is configured in such a manner that the light source portion is basically disposed inside and integrally with the device. A configuration to completely separate the light source portion and the liquid crystal display unit is not disclosed, and naturally, there is a limit of reducing the thickness and the weight of the device. Further, there is no disclosure about a liquid crystal display system configured in such a manner that illumination light is supplied to a plurality of liquid crystal display units from one or two light source portions. A further discussion is therefore necessary in order to achieve a system that supplies illumination light to the liquid crystal units that are reduced both in thickness and weight from one or two light source portions.    Patent Document 1: JP-A-2005-64163    Patent Document 2: JP-A-11-167808    Patent Document 3: JP-A-2006-134661    Patent Document 4: JP-A-9-159985